10 B.E.K. Klein

In summary, in adults of largely European background there appears to be a cohort effect on myopia.

Risk Factors for Myopia

Risk factors for myopia or myopic shifts in adults are given in Table 2. A description of these and other risk factors in children and in adults is given below.

Near work

Much of the information on the association of near work with myopia in children is inferred from estimated intensity of school work or reading. A study in Hong Kong examined fishing families and found an association between education and myopia.27 Hepsen and colleagues reported on greater frequency of progression of myopia in children from private schools as compared with apprentices in a skilled labor group.28 Saw and colleagues reported a significant association between the degree of myopia and the

Table 2. Selected Characteristics* Associated with Myopia in Adults

Abbreviations: SES = socioeconomic status; BDES = Beaver Dam Eye Study; VIP = Visual Impairment Project; LALES = Los Angeles Latino Epidemiologic Study; Inf. = inferred.

*Direction not given as associations may vary by strength and direction between categories of some characteristic.

**Near work was associated but not education.

+ Association found.

0 Association evaluated but not found.

11 Epidemiology of Myopia

number of books read per week in a group of Singaporean school children.12 In a study of Los Angeles and Australian 6- and 12-year-olds, parents’ report of children’s near work activity was modestly associated with myopia.9

Recently, Rose and colleagues reported a marked difference in the prevalence of myopia between Australian and Chinese Singaporean 6- to 7-year-old school children. The prevalence in Australians was 3.3% and in Singaporeans, 29.1%, despite the fact that the Australians read more books per week and did more hours of homework per week.29 The possibility that recent increases in years of preschool instruction for Singaporean children may be related to the higher prevalence in these children.

Khader and colleagues found that myopic children were likely to spend more time reading and writing and using the computer than their nonmyopic school mates,30 but the analyses were not adjusted for age, which is likely to be an important confounder in these analyses. Rah and colleagues in a study of myopia in parents and children have found that there is an association between near work and myopia, but speculated that the actual strength of the association was probably imprecise because of the inaccuracy of measures of near work. They suggest that better methods of reporting near work activities are needed for future myopia research in children.31

A relationship between near work activity and myopic change in refractive error has been found in adults. Microscopists have been shown to have higher prevalence of myopia than the general population and higher prevalence of adult progression of myopia, but a comparison group was lacking in this report.32 Studies of other specific exposure groups, e.g. medical students,14 suggest that these persons have greater prevalence of myopia than other similarly aged groups. Wu et al. reported that adults who reported near work activities were more likely to be myopic as compared with others in the population.16 Few studies in adults have had careful, precise measures of near work and therefore it is yet to be established whether near work activity is the important exposure and not just a confounder of other important possible causes.

Education/Income

Education and income are considered together because it is usually not possible to separate the effects of these two exposures. The association of more myopic refractive error with level of educational achievement (and usually with income as well) in children and adults has been found in most

12 B.E.K. Klein

studies of refraction.9,12,15,18,33,34 It is thought that this reflects near work activities, although there is a dearth of studies that assess the relationship quantitatively and by specific activity as noted above. The education/ refraction association relation may reflect common genetic determinants of intelligence (or educational achievement) and refraction.35 It is noteworthy that education was not associated with change in refraction in two large epidemiologic studies of adults.36,37

Outdoor activity

Ip et al. reported a small effect of hours spent outdoors on refraction (more hyperopic) in children in the Sydney Myopia Study.9 This finding was extended by Rose and colleagues, who reported on refraction in a sample of 6- and 12-year-old school children in Sydney, Australia. They found an inverse association of total time outdoors with refraction after adjusting for near work, parental myopia, and ethnicity.38 Hours spent playing sports was inversely associated with myopia in a study of 1777 students aged 12to 17-years old in Amman, Jordan, but these data were not adjusted for age.39

Jones and colleagues reported that lower amounts of sports and outdoor activities increased the odds of children, with two myopic parents, becoming myopic.40 The chances of children with no myopic parents becoming myopic was the lowest in the children with the greatest amount of sports and outdoor activities. Higher levels of total time spent outdoors, rather than sports per se, were associated with less myopia after adjustment is made for near work, parental myopia, and ethnicity. Rose and colleagues reported that Australian 6- to 7-year-olds spent more hours in outdoor activities than Singaporean children of the same age, the latter having a higher prevalence of myopia.41 Jacobsen and colleagues reported an apparent protective effect of physical activity for development of myopia over a two-year interval in a group of medical students in Copenhagen.42

There is no data to suggest that physical activity or sports has any effect on refraction or change in refraction in adults.

Age

In childhood, increasing age is associated with increasing prevalence of myopia.43,44 In adults, increasing age is associated with a hyperopic shift36 unless cataract is present when there may then be increasing myopia.45,46 The age effect is further described in the section on “Review of Studies.”